There are two answers
to the question how to manufacture air pollution control car.
First answer is to make ‘complete engine’ not to discharge harmful
gas, and the second answer is to ‘convert into harmless gas’
before harmful gas generated from the engine is discharged into
the air. Of course, now many engineers have made every effort
to the first answer(electric car ,hydrogen-fuel automobile etc.)
and a solution to the second answer is automobile catalytic
system being currently used. Automobile catalyst is divided
into three way catalyst(TWC) for gasoline fueled cars and oxidation
catalyst for diesel cars. TWC is named from the meaning of ‘simultaneous
reaction with 3 harmful elements’, and the three harmful substances
are CO, HC and NOx. These three substances have their unique
harmful characteristics; CO phagocytizes hemoglobin carrying
oxygen in blood, HC causes hallucination similar to bond, and
NOx causes dyspnoea by stimulating the mucous membrane of the
respiratory system. Therefore TWC is necessary for automobiles
not to discharge harmful exhaust.

The TWC system is installed between the exhaust manifold(collecting
exhaust from the combustion chamber) of the engine and the
noise suppresser in a long hexagonal shape. As if 'catalyst'
does not participate in a reaction but just promote it, ‘catalyst’
in the TWC system helps harmful gas to be readily converted
into harmless gas. As catalysts, several additives are added
to metal catalysts of 'Platinum'(Pt), 'Palladium'(Pd) and
'Rhodium'(Rh), and harmful exhaust is purified as it passes
through the honeycomb catalyst system. CO is combined with
oxygen into CO2, CH into CO2 and water, and NOx into nitrogen
and oxygen, then consequently nearly harmless gases(95% at
least of harmful gas is purified) are exhausted into the air.
For reference, water falls from the exhaust pipe of a car,
which implies that fuel is not leaked but exhaust is purified
properly.

Purification of the TWC system may differ according to the
‘concentration of oxygen’ in exhaust. Because the concentration
of oxygen may vary by 'air-fuel ratio', it shall be precisely
controlled to make the TWC system properly function. However,
existing carburetor-type engine(mechanically mixing air and
fuel) has limitation in controlling air-fuel ratio, thus current
electronically controlled engine has appeared. In conclusion,
electronically controlled engine has shown up to reduce harmful
exhaust.